constant anti spam questions

Hello
I have seen on some websites, to register you need to answer an anti spam question. like "what do you get when you freeze water?"
These questions are what I call "constant", meaning when you refresh the page, they dont change.
how can these be effective (I assume they are effective, otherwise, the webmasters would be doing something else)
I mean I have never used xrumer, but I would imagine you could easily train xrumer to put "ice" there.

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